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The B-1 had thin metal prop blades and no adjustable radiator flaps. It also had an open engine crank port, The B-2 had wide wood blades and adjustable radiator flaps. It had a cover over the crank port. The top of the radiator/underside of the cowl (where the lower part of the cowl formed the top) was curved on the B-1 and V shaped on the B-2. The B-1 was often retro-fitted with late B-2 style exhausts. . That's it.Here's a toothy grin!
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The B's and C's didn't have adjustable cowl flaps from what I've seen, they had shutters ahead of the cooler. Here's a B (note the Jerico trumpet and MG ports faired over) and you can see the oil cooler's cowl lacks the flaps. The first photo in this post is also a B...you can see the shutters closed in front of the oil cooler. I believe that they eliminated the shutters and replaced them with the flaps on the D series.
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